Frequency control system



Jan. 13, 194s. E. l.. @NUON V2,4.",4294

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FREQUENCY CONTROL SYSTEM l Filed oct. 22, 194,3 .4 sheets-sheet 2 INVENTOR 95 EDWARD L. G//vzTo/y fof BY Jail.l 13, 1948. v L., GlNzToN 2,434,294

FREQUENC-Y CONTROL SYSTEM Fned oct. 22, 194:5 4 sheets-sheet 4 INVENTOR EowA/o L G//vzTo/v ATroRNgY rPatented Jan. 13, 1948 FREQUENCY CONTROL SYSTEM Edward L. Ginzton. Garden City,l N. Y., assignor to Sperry Gyroscope Company, Ine., a corporation of New York Application October 22, 1943, Serial No. 507,235

' (cl. 25o-3c) 18 Claims.

This invention relates to high frequency electrical apparatus and more .particularly to automatic frequency control arrangements for such apparatus.

Pursuant to statutory requirements that the principle of the invention be explained in its best known mode of application, the invention will be described as applied to automatic control of the output frequency of a hollow resonator type ultra high frequency electron discharge device, but it will be understood that the invention is not so limited, as will be further apparent from the scope of the claims herein.

Considerable difliculty in the past has been eX- perienced in accurately controlling the output frequency of hollow resonator and like ultra high frequency devices, especially, for one example, in a receiver where the device is an oscillator to be maintained at a desired frequency difference from a received ultra high frequency wave for producing a iixed intermediate frequency utilizable for indication and like purposes. Also, considerable dimculty has been experienced in obtaining automatic frequency control in radio object detecting receivers and the like having pulsed inputs, and the invention contemplates a practical solution of this problem.

This inventiony also relates to an important phase thereof to automatic frequency control systems for reliably obtaining desired correlation between a plurality of different associated frequencies.

As disclosed in United States Letters Patent No. 2,294,942, in which I am a co-inventer, auto vmatic frequency control systems employing discrmnator action have heretofore -been proposed. In similar proposed systems employing frequency sensitive dscrimi'nators diiculty has often been encountered because the controlled oscillator at times may become stabilized at 'a frequency which is separated from the reference source frequency by other than the desired intermediate frequency, due to undesired discriminator action. The present invention is also concerned with elimination of such undesired discriminator action.

It is a vmajor object ofthe invention to provide novel, simplified apparatusA for attaining and maintaining reliable control over'the output frequency of a high frequencysource. Pursuant to this object, the source may be an ultra high fre'- quency hollow resonator device.

A further object Vof the invention is to provide novel, simplified electrically energized apparatus for varyingor scanning theoutput frequency of an ultra high frequency source, such as a hollow resonator device, cyclically over a predetermined range or band sweep. and for eventually maintaining said output frequency substantially constant or oating at or near a desired value within that range. Specically pursuant to this object, the electrically energized apparatus may preferably actuate thermally responsive arrangements adapted for variation of the natural frequency of `a hollow resonator device.

It is a :further object of the invention to provide novel control arrangements for cyclically actuating the tuning mechanism of a hollow resonator device to repeatedly tune said device over a predetermined range of frequencies.

A further object of the invention is to provide a novel pulse responsive automatic frequency control system.

A further object of the invention is to provide an automatic frequency control system embodying a novel multivibrator or like saw-tooth wave generator arrangement for producing cyclically varying tuning control energization, and for maintaining a hollow resonator or like ultra high 'frequency device at or near a desired frequency condition.

A further object of the invention is to provide novel frequency control apparatus for a hollow resonator `electron discharge device wherein an electro-mechanical tuning mechanism and an electron beam control electrode are actuated by control circuits having different time constants.

It is a further object of the invention to provide a novel automatic frequency control apparatus fora hollow resonator device wherein an electromechanical tunng mechanism 'and an electron beam control electrode are both responsive to the output of a multivibrator or like saw-tooth wave generator, thereby providing automatic frequency controls of different sensitivity. If the device is of the single resonator type, wherein an electron beam reector returns the beam into the resonator, the control electrode is preferably the electron beam reflector.

It is a further object of the present invention to provide a novel automatic frequency control system wherein a plurality of source frequencies are maintained substantially at a predetermined frequency difference.

A further object of the invention is to provide novel main and auxiliary discriminator arrangements in an automatic frequency control system.

A further object of the invention is to provide a novel automatic frequency control system.

source is controlled by novel discriminator ar. rangements.

It is a further object of the invention to provide improved apparatus in a fixed frequency difference system for preventing the controlled frequency from "locking-in" on the wrong side (for example, above instead of below) the reference frequency.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims` and the annexed draw- I lnator output responsecorresponding to changes detection and location, and embodying the invention. Pulses of ultra high frequency energy are sent out by a transmitter 1 and, after reflection from an object, are returned to receiverg wherein they are utilized to indicate the range, direction and character of the object.

The received pulses are ultra high frequency waves and are introduced into a mixer 9, along with ultra high frequency energy from a local oscillatorv II. The intermediate frequency output of mixer 9 is then amplified at I2 and supplied to a suitable indicator circuit or device I3.

in local oscillator frequency in the system of Figure 1I Figure 8 is a sectional View illustrating a mul-i tiple hollow resonator device optionally controllable by the circuit of Figure 1;

Figure 9 is a partly diagrammatic view and wiring diagram illustrating a further embodiment of the invention, substantially adding a short time constant automatic frequency control to the arrangement of Figure 1;

Figure 10 illustrates diagrammatically an automatic frequency control system according to the invention, but having a continuous wave input:

Figure 11 is a diagrammatic view illustrating a further embodiment of the invention in an ultra high frequency control circuit;

Figure l2 is a partial block and wiring diagram illustrating an automatic frequency control with multivibrator scanning but not requiring pulsed operation;

Figures 13-16 are graphs illustrating operation of the system of Figure 12;

Figure 17 is a partial diagrammatic View illustrating a further embodiment of the system of Figure 12; y

Figure 18 is a partially diagrammatic illustration of a further embodiment of my automatic frequency control invention;

Figure 19 is a graphic illustration of the main and auxiliary discriminator response characteristics employed in explanation of the frequency control exercised by the invention of Figure 18;

Figure 20 is a further explanatory graph illustrating the nature lof the frequency control of the invention of Figure 18; and

Figure 21 is a diagrammatic view illustrating a control circuit similar to that of Figure 18 but employing a simple tuned circuit instead of an auxiliary discriminator for controlling operation of the main discriminator.

Ultra high frequencies, within the meaning herein employed. relate to frequencies in the order of 3 108 cycles per second or higher. Also the designation hollow resonator includes any cavity resonator wherein the interior is evacuated or contains any dielectric medium in gaseous, solid or liquid form.

Figure 1 illustrates a pulse transmitting and receiving system such as is used for radio Object It is important to maintain a substantially fixed frequency difference between the received waves and the output of oscillator II, so as to maintain the intermediate frequency outputof mixer 9 substantially constant at a desired frequency value for utilization.A Hence, in Figure 1, I provide an automatic frequency control for oscillator which is responsive to variations in the intermediate frequency from that desired value, so that variations in both the received and' oscillator frequency ,may be compensated for 'in the system by suitable regulation of oscillator II.-

To this end, part of the intermediate frequency output from amplifier I2 is introduced into a suitable frequency or phase sensitive discriminator I4 having an output polarized according to the direction of departure of the intermediate frequency from the desired constant frequency. Discriminator I4 may be tuned to the desired intermediate frequency output of mixer 9 or may for example be the same as that disclosed in United States Letters Patent No. 2,294,942.

The output of discriminator I4, amplified if necessary, is fed into a suitable controlv circuit I5, which latter forms a major part of the invention and is shown in detail in Fig. 2. Controlcircuit I5 is connected through a suitable power Acircuit I6 to effect frequency regulation of oscilchiefly accomplished by maintaining the output frequency of oscillator II substantially at a desired value.

Figure 2 illustrates' details of control circuit I5 and the controlled local oscillator II employed in a preferred embodiment of the invention according to Fig. 1. Due to the pulsed nature oi' the wave received by receiver 8, the output of discrlmlnator I4 will be in the form of pulses. The amplified pulsed output of discriminator I4 is connected across input terminals I1, I8 of control circuit I5 ywhich is essentially a` multivibrator circuit having special control features and novelty in the illustrated combination.

Control -circuit I5 comprises a. vacuum tube multivibrator circuit embodying two triodes I9 andv 2| having their respective anodes 22 and 23' connected through respective resistors 24 and 25 to one terminal of a common unidirectional current source 26 whose other terminal is understood to be grounded. Cathodes 21 and 28 of the respective triodes I9 and 2| are connected to acommon grounded lead 29 to which is also connected terminal I8. Terminal I 'I i: connected to grid 3| of triode I9 through a blocking or coupling condenser 32 which isolates the multivibrator circuit from the unidirectional output of the discriminator I4. Grid 3I is connected to ground lead 29 through a grid leak resistor 33.

and to anode 23 of triode 2| through condenser 34. Grid 35 of triode 2| is connected to groundv 6,A necessary to keep direct current from power circuit 'I5 from affecting operation of control circuit I5.

As shown in Figure 2, the output of control circuit I derived across condenser 4I is connected to control operation of the oscillator power circuit I5, whose'output lead 58 feeds power to Oscillator comprises a conventional type single hollow resonator device having a cylin drical reentrant hollow resonator 42 formed with a rigid wall 43 and a flexible annularly crimped wall 44 at opposite ends. Wall 43 is apertured for mounting an exit grid 45, and wall 44 is also apertured for receiving a hollow reentrant pole 45 carrying an entrance grid 41 parallel to grid 45 on its inne;` end. Annular flanged members 48 and 49 are rigid with wall 43 and pole 4|.

A cathode 5I, heated from a suitable source, is connected tothe negative terminal of a unidirectional acceleration voltage source 52 having a grounded positive terminal, for projecting a stream of electrons through the oscillatory resonator neld between grids 41 and 45. A reflector electrode 53, maintained at a' slight negative or positive potential with respect to cathode 5|, is located to return the electron beam into the space between resonator grids 45 and 41'. Cathode 5|, grids 45 aligned in the device in the electron beam path. The parts are so dimensioned and energized that the electron beam excites and maintains ultra strut 55. According to the invention the power delivered over lead 58 varies exactly with the cyclically varying output voltage of circuit I5 derived from condenserll.

Output lead 58 from power circuit I5 is preferably connected to the midpoint of strut 55, so

that both halves of strut 55 are electrically in parallel, flanges 48 and 49 being directly connected by the metallic shell of resonator 42 and grounded. If desired, however. strut 55 may be heated by a separate heater coil therewlthin, or in any other desired manner.

Strut 55 expands in length substantially in proportion to the output of power circuit i5. which follows the current strength of the multivibrator output, thereby correspondingly varying the spacing of grids and 41 and the natural frequency of the oscillatory circuit 'vithin resonator 42, for purposes to be explained.

Operation The system of Figure 1 is energized and each element rendered operable. If the intermediate frequency from amplifier I2 is at the desired conand 41 and reflector 53 are all axially high frequency oscillations in the resonator, so

that ultra high frequency energy may be available for extraction therefrom. Operation of such a hollow resonator device as an ultra high frequency generator is known and described inthe United States Letters Patent No. 2,250,511, to which reference is hereby made for further detail. e

Glass closure caps 54 and 55 seal off opposite ends of thev resonator device so that the interior may be maintained evacuated. Concentricline 51 isprovided for connecting resonator 42 to mixer 9 to supply high frequency energy thereto. Tuning of oscillator II is accomplished in this embodiment by controlling the relative spacing of grids 45 and 41. A rigid strut 55 extends betweenanges 45 and 49. Strut 55 is preferably a thin-walled hollow tube of duralumin or some material having a relatively high coeicient -of thermal expansion.

' Looking into the output of control circuit I5,

, a high impedance is observed,` while strut 55 on the other hand is obviously of a relatively low im.. pedance. Moreover, the output power of the multivibrator circuit may not be suiciently large to provide the power required for energlzation of strut 55. The purpose of power circuit I5 is to convert the variable output of control circuit I5 into a similarly, preferably identically. varying highvpower output for energizing strut 55.

Preferably power circuit I6 is a conventional oscillator system,vthe output of which varies identically with the voltage across condenser 4|, and

which supplies enough power for suitably heating the strut. 'Since the details of such oscillator are not part of the present invention, further description thereof is not necessary. Sucha power circuit is shown, for example, in Fig.`8,oi application Serial No. 486,589, filed May 11, 1943, in the name of H. M. Stearns. Condenser 40 is provided where stant value, or is outside the frequency range to which discriminator I4 is responsive, there is no control signal from 'discriminator I4 and the output frequency of resonator 42 is variably controlled by the multivibrator circuit alone.

Figures 3, 4, 5 and 6 illustrate the operation oi" the multivibrator utilized in my invention, which may be suitably energized by connection to source 25. 'Ihe multivibrator circuit will initially be in a state of unbalance with either grid 3| or 35 more positive, since the circuit is inherently unstable, as is known.

Assume that for some reason, grid 3| of triode I9 is initially made more positive than grid 35 of triode 2| in any desired manner, as indicated in Fig. 2. The plate current of triode I9 is thereby increased, lowering the plate voltage and causing grid 35 of triode 2| to become more negative. The plate current of triode 2| is thereby decreased, raising the plate voltage thereof to make grid 3| morepositive. The above continues until within a very short space of time grid 35 is so highly negatively charged as to block triode 2 I, triode I9 being then at maximum conductivity. This is the condition represented at the left end' of Figures 3 and 4 showing the variation of anode current for triodes I9 and 2| respectively, the condition prevailing for the period indicated as t1.

During the period t1, however, the high negative charge on grid 35 leaks off through grid leak resistor .35, and eventually grid 35 becomes more positive than cut-off and starts to conduct. An increase in conductivity of triode 2| makes grid 3| more negative and reduces the conductivity of triode I9. This reverse action now continues until triode |9 becomes blocked and triode 2| is fully conducting, a condition prevailing during the period t2 in Figs. 3 and 4L The above is repeated cyclically, and triodes I9 and 2| become alternately conductive and nonconductive as illustrated at periods is, t4, etc., in Figures 3 and 4. The speeds ofthese cyclic operations and the lengths of periods ti, t2, etc., are determined by the values assigned to grid condensers 34 and 31, andgrid leak resistors 33 and 36. Preferably the periods are made about equal for purposes of the invention, for uniform scanning control. The above described multivibrator action follows accepted theories of operation of such circuits; The output between tap 39 and ground is of square wave form, and filter circuit 38, 4I is employed to integrate the square wave output and obtain the cyclically varying control voltage output shown in Figure 5.

Energy from source 26 is effective to charge condenser 4I during the periods t1, ta, etc., when triode 2| is blocked; and condenser 4`I discharges during the periods t2, t4, etc., when triode 2| is conductive. Figure illustrates the resultant cyclic variations in voltage V41 across condenser 4I, and therefore also illustrates the resultant variations in power output of power circuit I6.

The cyclic output voltage of condenser 4I controls alternate heating and cooling of strut 56, although the thermal inertia of strut 56 may cause the resultant elongation and contracting of strut 56 to slightly lag the condenser voltage variation.

As strut 56 alternately elongates and contracts, it correspondingly varies the spacing of grids 45 and 41 and the output frequency of resonator 42. The output frequency of resonator 42.increases when grids 45 and 41 are moved apart by elongation of strut 56, and decreases when the grid spacing is reduced by contraction 0f strut 56. The limits of V41 are so chosen that resonator 42 is repeatedly automatically tuned back and forth over substantially its entire operable frequency range. This tuning operation is also known as scanning of the resonator frequency range, and as pointed out above, takes place uniformly in the absence of any signal from discriminator I4. An important feature of this scanning control is that the cyclically varying voltage output from condenser 4I causes positive similar cyclical variation in the heating power applied to strut 56,' so that the resonator frequency is under positive control during the entire scanning time.

Hence, uniform scanning is immediately originated when the system of Figure 1 is energized, and continues until modified by the intrnduction of a discriminator output signal on terminals I1, IB, as will appear.

During scanning, the output frequency fo of resonator 42 decreases during periods t1, t3, etc., when condenser 4I is charging and the energization of strut 56 is decreasing, permitting strut 56 to cool to reduce the scanning between grids 45 and 4l, and increases during periods t2, t4, etc., when condenser 4| is discharging and strut 56 is increasingly energized to elongate to increase the grid spacing. Figure 6 illustrates, at the left side, the manner in which the resonator output frequency fo varies theoretically during scanning, although actually it may lag somewhat due to thermal inertia of strut 56. In any event, however, the Afrequency variation is cyclic and regular during scanning.

Assume now that the output frequency of resonator 42 is such that the intermediate frequency output of amplifier I2 cornes Within the discriminator response range, for producing a unidirectional control signal across terminals I'l, I 8. Referring to Figure 7, which illustrates the response of discriminator I4l to changes in frequency 0f oscillator II, it is noted that there are two values of local oscillator frequency fo which may combine with the transmitter frequency ft to produce the desired intermediate frequency fa.' lIfhe discriminator response at the lower of these two oscillator frequencies fi is the mirror image of the discriminator response at the upper oscillator frequency f2. This is because the intermediate frequency varies in the same direction as the oscillator frequency for values of fo above fr, and varies oppositely to the oscillator frequency for values of fo below ft.

The polarity of output of discriminator I4 may be reversed by reversing the output leads as is known. In the described arrangement, the discriminator output is selected polarized as shown in Figure 7 for Vthe operation to be described.

Assume now a. time tx when the multivibrator scanning action is increasing fo from appreciably below f1. As shown in Figure 5, during this period tx, condenser 4I is discharging to cause power circuit I6 to increasingly energize strut 56. When fo becomes such that the intermediate frequency enters the range of discriminator I4, a

negative signal from the discriminator is im pressed on grid 3|. This signal is of course made up of a succession of pulses which may or may not be regular or periodic, but which are sufilciently close together to afford control action.

Now, during the period ,fo is increasing, tube I9 is non-conductive, with grid 3| already highly negative; This means that the negative discriminator signal has no eiect on the 'multivibrator scanning action.

However, when fo increases beyond f1. the discrimnator output reverses in polarity and a positive signal from the discriminator is impressed on grid 3|. This signal comprises a succession of positive pulses. As soon as a positive pulse of sufficient amplitude is delivered to grid 3l, the above described multivibrator scanning action is interrupted andvmodied. The positive discriminator output. pulse renders grid 3l positive so that triode I9 becomes conductive. As a result, triode 2| becomes blocked, and discharge of condenser 4I is interrupted to thereby cause power circuit I6 to reduce energization of strut 56.

The increase in oscillator frequentir fo is therefore halted just above f1, and the above described premature forced reversal of the multivibrator circuit causes the oscillator frequency to now start decreasing. The decreasing oscillator frequency soon drops below f1, at which time the discriminator output reverses to negative. As soon as a negative pulse of sufficient amplitude is delivered to grid 3|, triode I 9 is again blocked. As a result triode 2| becomes again conductive, and charging of condenser 4I is interrupted to thereby cause pow/er circuit I6 to increase energization of strut 56. Decrease in the oscillator frequency is thereby halted. and increase in oscillator frequency again started.

The output of discriminator I4 thus modifies the multivibrator scanning action. The fact that the discriminator output is pulsed aids automatic frequency control, because the multivibrator circuit is'of such nature that a signal of pulse duration only,v of the proper polarity and amplitude, is sufficient to trigger the above-described reversals in operation. Moreover, no periodicity or regularity of theA pulses is necessary.

As shown at theright end of Figure 5, the above-described discriminator action causes the voltage across condenser 4I to fluctuate or float closely about a medial value determined by the discriminator operation. The power output of power circuit I6 follows these voltage fluctuations. As a result, it is clear that the discriminator output exerts very close positive control and` substantial stabilization of the resonator output frequency, andV consequently the intermediate frequency. The frequency response of the illustrated resonator device is.l as pointed out above, subject to thermal `inertia so that as a practical matter the resonator frequency does not `fluctuateas sharply or to the same degree as the control power as indicated-at the rightln Figure 6. Inefrect, the discriminator action maintains the resonatoroutput frequency substantially floating about a frequency selected for producing the ie--l sired intermediate frequency Vwithin permissible limits.. so that the single control circuit I automatically maintains a substantially constant in-A termediate frequency fa for utilization as at indicator I3. l 4

`When the frequency of resonator 42 has a tendency to drift during operation, as. for v`exam- While I have described controlv circuit I6 as 'actuating a thermen strut, it 1s within the scopelof the invention to substitute for strut 5B any electro-mechanical device such as a motor, piezoelectric member, or magnetostrictive member suitably connected to flanges 48 and 49.

l0 and spaced output grids 68, '69, in the walls of resonator 63.

Grids 6B and 88 are flexibly connected to'the i resonator body. as by flexible resonator .end walls 1i and 12, and are lrigid with radial flanges 13 and 14. Flexible ribbons or wires 15, of some conductive material --having a high coeiilcient of thermal expansion, are secured at opposite ends to a flange 10 rigidA with the resonator and to flanges 'I3 and 14. Suitable compression springs 16 maintain wires 15 under tension.v

Leads in Figure 1 is connected to the mid- 'points of wires 15, for accomplishing automatic frequency Vcontrol similar to Figure 1 above described; Gtherwise, the operation of device 8l as anoscillator is conventional. and similar tov that described in United States Letters. Patent No. 2,242,275. Strut 56 or wires 15 may be interchangeably used in the structures of Figures 2, `8 and 9. Care only is needed to cprrelatethe control action with the relation between the expanding strut or wire and frequency.

Figure 9 illustrates ,an automatic frequencycontroLcircuit which is similar to Figure 1, but includes also a relatively fast automatic electrical tuning control synchronized withthe electromechanical tuning control. As in Figure 1, the output of condenser 4i supplies cyclic energization of strut 56. Due to thermal inertial,` of the strut, circuit elements 38, 4 i, as` above pointed out, necessarily are selected .to have a relatively long time constant, so that the strut may be properly and efficiently energized for maximum con- Furthermore, while the invention is above de- `scribed' as adapted for maintaining a desired intermediate frequency -in the illustrated system,

it is obviously equally well adapted for maintaining thev output frequency of anyk device, such as hollow resonator 42, substantially fixed. For example, any constant controlled frequency source could be utilized ,inl place of receiver 8, as will be explained in connection with Figure 10, or an arrangement similar to Figure 11 may be employed. Resistor 38 and condenser 4I comprise a relatively long time constant filter circuit for integrating the normally substantially square .wave output -of the multivibrator circuit. If desired, for some installations as where the thermal inertia of strut 56 is high, this rlter may be eliminated to provide direct actuation of the power circuit by the multivibrator.

As pointed out above, the outputof discriminator i4 is pulsed because the received waves are pulsed. Far frominterfering with the above desidered as very useful to the above described cyclic control of` the multivibrator circuit. As above explained, the oppositely polarized pulses in the discriminator output positively tend to drive the oscillator frequeneyback and forth cyclically within limits which may be determined by suitable circuit constants.

Further embodiments control, I provide relatively small time constant trol. As in Figure 1, the power output of the con-f' trol circuit may be stepped up by `use of apower circuit similar to that at I6 in Figure 1.

For correlated auxiliary automatic frequency circuit elements comprisingY series connected rcsistan'ce I1 and condenser 18 arranged in parallel with elements 38, 4l across the output of triode 2 l. A lead 19 is connected to refector electrode 53 so that the potential of reector electrode 53 varies cyclically in exact periodic synchronismwiththe, control action illustrated in Figure 5, but ata faster control rate since\t\here is no thermal inertia to overcome. Since varialtion in the potential of reflector'53 alone may control the resonator frequency as known, it is apparent that automatic frequency control obtained by the combined arrangement of Figure 9 is rendered immediately effective due to the shortv time constant circuit elements, and posi i tively and 'accurately regulated by the long time constant circuit elements 38, 4I which determine the periodicity of the scanning operation and scribed operation, this pulsed inputY may becontor device 6 I which may be employed as the oscillator Il in Figure 1. Device El comprises a pair of hollow resonators 62 and 63, adapted to contain oscillating electromagnetic fields which are coupled together by loop 64. A cathode 65 is provided to proj ect an electron beam through spaced input grids ,66. 61 in the walls of resonator 82,

take care of large frequency changes. The multivibrator circuit in Figure 9 differs from that of Figure 1 also by the inclusion of grid resistors and 80' in series with condensersli and 31 respectively, for reducing the negative grid swing for each triode and thereby speeding up tripping of the multivibrator. This phase of the invention, wherein long and short time constant controls are employed for electro-mechanical and electric tuning members respectively, is of course not limited to automatic frequency control. but may be embodied in a. manually controlled frequency regulation system as where the two R-C circuits 38, 4i and 11,. 18 are connected across a manually variable power source.

` Figure 10 illustrates an automatic frequency control vsystem according to the invention,

wherein acontinuous wave reference source is high frequency input to mixer 9. Y Discriminator I4 and control circuit I5 are the same as in Figure 1 or 9. Inorder to provide pulses for oper-l ation of the multivibrator circuit as above explained, I connect a suitable pulse generator 32 to amplifier I2, but any equivalent manner of obtaining a pulsed discriminator output lies within the scope of the invention. The useful output from oscillator I I is extracted by a coaxial conductor or a wave guide indicated at 83.

Figure 11 illustrates a further system embodying the invention wherein the output frequency of ultra high frequency oscillator II is directly controlled through external apparatus suitably connected thereto. Oscillator II is provided with a suitable concentric line 84 for extractng energy at the controlled frequency for any desired utilization.

Output energy from oscillator II is introduced into a blocking amplifier 85 to which is connected a pulse generator 86. Amplifier 85 and pulse generator 86 may be of any conventional type. and the chief purpose of their association herein is to pulse the output of oscillator II prior to input to an ultra high frequency discriminator 81 connected to amplifier 85. Ob-

viously any equivalent arrangement for producing a pulsed output from oscillator II may be employed. Control circuit I5 is connected to receive the output of discriminator 81 and to effect frequency control of oscillator II, similarly to Figure 1 or 9.

The dotted line in Figure 11 indicates any suitable electrical or mechanical control link between the control circuit I5 and oscillator II. It may comprise frequency control by motor driven tuning device as in Patent No.l 2,294,942, or by the abovemethods described in Figs. 1 and 9.

In the above described embodiments of the, invention automatic frequency control is attained by connecting the discriminator to modify the scanning operation of the multivibrator circuit. Figure 12.il1ustrates a system wherein the multivibrator circuit at all times tends tov exercise frequency range scanning and wherein the discriminator is connected in parallel with the output of the multivibrator circuit so as to modify scanning and maintain substantially a desired intermediate frequency during periods when the discriminator is energized.

Referring now to Figure 12,'crystal controlled ultra high frequency source Bland the controlled oscillator II are connected to mixer 9, and the intermediate frequency output of mixer 9 is amplified at I2 and introduced into discriminator I4, similarly to Figure 1. Source 8| may be of relatively low power as compared to oscillator I I, and may be of a known type accurately maintained at a selected frequency, for example, by suitable crystal control and a frequency multiplier chain.

`The polarized output of discriminator I4 is connected to grid 85 of an amplier 96, having its anode 81 connected through a suitable output v resistor 88 to a source 89, and having a grounded cathode 9|. Anode 81 is also connected to energize'the frequency control member of the controlled oscillator, as for ',example, a thermal tuning strut such as 56 in Figure 1.

A suitable scanning control circuit 93 containing a multivibrator circuit 94 similar to that described in Figure 2 is also provided with an output resistor 38 and condenser 4I as in Figure 2. Lead 99 from the multivibrator output is here connected to. the grid .95 of a trlode 96 having its anode 91 connected through resistor 98 to a suitable power source 99', and having a grounded cathode IIII. Anode 91 is connected to the frequency control member of oscillator II along with the discriminator output. It will be noted that the discriminator output is here not fed into the multivibrator circuit as in Figure 2, and hence does not modify operation of the multivibrator circuit. but is used to modify the multivibrator 10 output. Arrow I1 indicates control current due to the scanning control circuit, and arrow I2 indicates control current -due to discriminator action, while the other arrows indicate direction of flow of the control iniiuences.

In operation, upon energization, control circuit 93, through the multivibrator action above described, produces control current I1, which tends to vary cyclically, as shown in Figure 13, between selected limits. so as to cause similar cyclic variations in the output frequency fo of oscillator II as indicated. This operation initiates scanning of oscillator frequency range and would continue indefinitely were it not for the below described control action immediately exercised by discriminator I4.

As current Ii increases, this tends to increase the oscillator frequency, and consequently the intermediate difference frequency. Figure 16 illustrates the frequency response curve A of discriminator I4 and shows that the discriminator output reverses in polarity yas the intermediate frequency increases through the desired frequency fd. Figure 14 represents current I2 produced by discriminator action. Current I2, being produced by changes in frequency fd caused by variations of I1, has the same periodicity as I1, but is opposite in direction because of the inverse discriminator response. Discriminator I4 is tuned to desired frequency fd, which is the midfrequency region of the frequency response of amplifier I2 represented by curve B.

The slope of curve A passing through fd is so chosen that variation in the intermediate frequency near frequency fd and over the relatively small frequency range 2Af will produce a relatively large change C-D in amplitude in the discriminator output. The slope of curve A at C-D can be made as sharp 'as desired by suitable amplification of the discriminator output, and the circuit elements are so designed that a change in scanning current I1, which produces a correspondingly small change in intermediate fre- -quency, will cause a proportionatelyvlarge dis criminator routput change and a control current Iz of opposite sense than and substantially equal amplitude with current I1.

Thus, as variation in scanning current I1 tends to cause the frequency of oscillator to sweep in either direction, opposing current Iz produced by the resultant strong discriminator output quickly halts that .frequency sweep after only a small change in frequency. The output frequency fu of oscillator .I I, and hence the intermediate frequency, thus oat between relatively narrow o5 limits. The average intermediate frequency is,

of course fa.

In the invention, the circuit constants are such that the intermediate frequency is theoretically maintained floating about fd between the limits ,ji/Af on either side. Actually a more nearly constant fd is maintained because of thermal and other inertia in circuit elements such as strut 56.

By tuning discriminator I4 to the desired intermediate frequency fd, I insure that the control discriminator' output.

'action takes'place uniformly about the frequency fa. The control action does not tend to take place at frequencies corresponding to slopes of the discriminator'response curve indicated at :r and y because, as shown in Figure 16, the discriminator output then produces a current In, which' is in the-same sense as '11, and actually speeds up the ,frequencysweep toward the control region C-D.

Should the reference or oscillator frequencies so change that no discriminator output is available. or if for any other reason there yis no discriminator output, the multivibrator circuit continues scanning control. Obviously any tendencyr for the oscillator or reference frequencies to drift, when the discriminator is exercising control, is

1 compensated for automatically.

Figure' 17 illustrates certain refinements on the system of Figure 12. which may be used if desired.` The usual three terminal frequency sensitive discriminator output is illustrated in Figure 17 as bridged by a large condenser |03. This condenser A is used only when discriminator I4 is connected to a pulsed input such as receiver 8 of Figure 1. Iand serves toleliminate pulses from the utilized In addition Figure `17 illustrates a further auxiliary control by which the output of discriminator I 4, amplified at |04, is employed to control a 'switching circuit |05 in the multivibrator out- Whenever the resonator frequency then produces an intermediate frequency outside thediscriminator range so that the auxiliary switching circuit is deenergized, the multivibrator circuit resumes scanning action to bring the resonator and locating system 14 utilizing the reference source as a transmitter oscillator. Also. the reference source may be a received wave, and the controlled oscillator may then be the local oscillator of a superheterodyne receiving system.

Source II is designated the controlled source because it is provided with arrangements as in Figure 1 for varying Vits operating or output frequency.V

The output frequency of source II. is selected to be near the reference frequency, being separated therefrom by the difference frequency appearing in the output of mixer l. It is this difference frequency. which is known also Aas the intermediate frequency. that is utilized for indicator, control and like purposes in the` system. Mixer 0 may therefore be regarded as the source of difference frequency. :The invention maintains this difference frequency substantially at a desired value, regardless of deviations in either tl reference or controlled oscillator frequencies.

The difference frequency output of mixer 0 is amplified at I2, and part of theoutput of ampliiler I2 is introduced into a,main discriminator IL Discriminator I4 is preferably of the frequency sensitive type indicated in Figure 1 having circuits, tuned to a selected frequency, in this case the difference frequency which is desired to be maintained constant, and producing output unidirectional voltage control signals polarized according .to direction of departure of the difference frequency from the desired constant' value.

The output of discriminator Il is introduced into a suitable control circuit I5 such as that of l Figure 2 operatively connected, as indicated 'by dotted line |08, to the frequency regulation mechanism in controlled oscillator I I. Dotted line |00 represents any suitable electrical or electromechanical connection between control circuit IE and the tuning or frequency regulating mechafrequency back within the control range of the discriminator.v Any of .the features above described with rreference to Figures 9, 12.and v17 may be interchangeably used between the systems of Figures 1; 9 and 12. e

Lock-in control In the phase of the invention illustrated in Figures 18-21, the frequency ofan oscillator or like source is maintained at a substantially fixed intermediate or difference frequency on a desired side of the frequency of a reference source. The invention is .especially adaptable to the automatic frequency control of ultra high frequency devices of the hollow resonator type, as will appear. The terms source as'used` herein is relative, and is not limited to generator or like original sources.

'Referring to Figure 18, which shows this phase ofthe invention as applied to systems similar to those of Figures 1-1'1, energy from areference source I 08 and controlled oscillator source II is introduced into Cmixer 8, which may be the same as the mixer-detector disclosed in Patent. No.

` nis'm lfor oscillator I l.

f tor Il and control circuit I5.

Triode IIS is provided with alsuitably biased The invention contemplates a refinement of control by the use of an auxiliary discriminator or equivalently functioning arrangement which is so cooperatively associated with any of the above described systems as to provide an eilicient and practical frequency control.

. In the embodiment illustrated in Figure 18. a part of the diierence frequency output of ampliiier I2 is introduced into auxiliary discriminator III, which may be the same as discrlminator I4. Discriminators I4 and III may be any conventional frequency sensitive discriminators such as is used for example in kno'wn frequency modulation receivers. Alternatively, auxiliary discriminator III may be any type of circuit whose output varies with frequency. One alternative form will be described belowwith respect to Figure 2l.

. The output of auxiliary discriminator III passi -A lead II'I connects resistor IIB to the-grid II! of a triode I I9 situated between main discriminacathode I2 I, an anode |22 and a loadresistor |23.

- Input terminals II, I8 of-controlcircuit I5 are connected across the output of triode I2 I.

The vpurpose of these arrangements in .con

15 tributing to automatic frequency control, will be explained below.

In operation, the auxiliary discriminator primarily functions to prevent stabilization of the system when the vzreference and controlled source frequencies are separated by other than the desired intermediate frequency. The following discussion of thaction of main discriminator I4 and the control problems present therein will aid in understanding the invention.

Referring to Figure 19 in which the solid line curve illustrates the frequency response of main discriminator I4, lt will be observed that discriminator I4 produces zero output at the desired difference frequency Fd; and that the output of discriminator I4 is positive when the difference frequency is below Fd. and negative when the difference frequency is above Fd. For purposes of explanation, also it will bev assumed that when the output of discriminator I4 is positive, the operating frequency of `oscillator I is caused to increase,

' and when the output of discriminator I4 is negative, the operating frequency of oscillator is caused to decrease, by reason of the discriminator control action effective through circuit I5.

In Figure '20, curves |25 and |26 show the` ef-` fective response of main discriminator I4 over a 4Should the increasing oscillator frequency reach and pass frequency f4 without being halted, frequency f4 representing maximum negative out.

oscillator frequency. Thus, after the oscillator controlled oscillator frequency range including the reference source frequency fr and frequencies fi and fa at either of which latter oscillator II I \may become stabilized for producing the desired difference frequency. When the frequency of oscilator II is below fr, an increase in the oscillator frequency causes a decrease in the resultant difference frequency. Hence curve |25 is the reverse of the solid line curve of Figure 19. When the frequency of oscillator is above fr, an increasein the oscillator frequency causes an increase in the resultant difference frequency. Hence curve |26 is similar to the solid line curve of Figure 19. Curves |25 and |26 are mirror im-v ages of each other, the axis of symmetry being frequency fr.

Consider first the condition that, when operation is initiated, the frequency of oscillator II is appreciably below f1 and increasing either from` y drift or by the scanning action described above with reference to Figures 1-17. This means that the difference frequency is initially very high and is decreasing. Referring toFigure 20, as the increasing oscillator frequency causes the decreasing difference frequency to enter the discriminator response, region, a negative output is first put forth by discriminator I4. But as above specilied, a negative output from discriminator I 4 results in a tendency to decrease the oscillator fre quency. As a consequence, unless corrective measures are taken, the increasing operating frequency of oscillator I2 is halted as soon as the negative discriminator output becomes suiiiciently strong, as for example when it reaches the point indicated arbitrarily at |21. The oscillator frequency is therefore prevented from rising above a frequency fa, and theabove rebuing action takes place every time either scanning action or drift tend to so increase theoscillatorfrequency. As a frequency Vincreases above f4, discriminator I4 exercises no tendency to haltsuch increase below fr, at which the difference frequency is zero.

As the oscillator frequency increases above fr, the resultant difference frequency now increases, and a positive output from discriminator I4 is first encountered when the increasing difference frequency enters the discriminator response band represented by curve |26. This is effective only to further increase the oscillator frequency. However. as the increasing oscillator frequency goes above f2, the output of discriminator I4 now reverses to negative and is 'effective to cause a decrease in the tendency of the oscillator frequency to increase. When the negative discriminator output becomes sumciently strong, as at point |28, the tendency to'drift is completely cancelled and the frequency remains substantially constant at fe. If the tendency to drift reverses, then,

`as soon as the now decreasing oscillator frequency passes below fz, the discriminator output again reverses to positive and is effective to oppose the tendency in the oscillator frequency to decrease. When the discriminator output becomes`4 sufliciently strong, as atpoint |29, the tendency to increase is' completely cancelled, Aand the frequency remains constant at fs. The frequency of oscillator I I is thus always maintained between limits island fs so as to be substantially at or near fz, thus maintaining the resultant difference frequency at or near the desired value. By suitable design of the control circuits, to provide a large frequency-correction for small discriminator outputs, these limits may be held'very close, and the frequency is maintained substantially constant for all practical purposes.

onsider, secondly, the opposite condition that when operation is initiated, the oscillator fre.. quency is appreciably higher than f2 and decreasing either by drift or by scanning action. This decreasing oscillator frequency tends to pass result, the oscillator frequency will tend to seek some relatively fixed level at or below fa, and the desired diierence frequency is unobtainable.

Before discussing corrective measures for the above dimculty, further possible control action of discriminator I4 will be considered to ascertain lf any further similar difficulties are encountered.

through f2, the discriminator output reverses to positive, and the above-described control action takes place for maintaining the oscillator frequency stabilized at f2 and hence stabilizing the' difference frequency at the desired value.

But, suppose the decreasing oscillator frequency should drop suiciently below f2 to avoid the immediately above described stabilizing action. Now, as the decreasing oscillator frequency passes ,fr and approaches f1, the resultant increasing difference frequency is effective to first cause discriminator |4'to have a positive output, tending to increase, or oppose the decrease in, the oscillator frequency. When this positive discriminator output becomes suiiciently strong, as at point |3|, the oscillator ceases to decrease its operating fre.- quency, so that the oscillator frequency is prevented from going below f1. If the tendency to 17 drift is reversed, so that the oscillator frequency increases, increasing os'clllatorfrequency then becomes stabilized at or near f2, as above described.

Should the decrease in oscillator frequency con tinue, so that it passes below fa, at the region of maximum positive output of discriminator I4. the less positive discriminator output encountered between fa and fi, and the negative discriminator output encountered below f1', would not interfere with and would assist such decrease.

Where the variation on oscillator frequency is caused by a scanningactlon, which periodically reverses the direction of frequency change, it will be seen that where the scanning starts below frequency f4, the frequency can'never exceed fa, but will vary periodically with j: as its upper limit.

.In this case, the desired diiferencefrequency fd is never reached. v

When the scanning action starts above fz, the oscillator will "lock-in at f2 where it first reaches that frequency,` and will remain there provided the scanning action is interrupted or is otherwise' ineective to overcome the control action of discriminator I.

When the scanning action starts between ,fa and f2, decrease in frequency will halt at f1. Subsequent reversal of the scanning action will increase the frequency to f2, at which lock-in will again occur. 1n each of the latter two conditions, the desired difference frequency is reached and maintained.

Hence, in the second condition where the oscillator frequency is decreasing, from above fr, or in the third condition where the frequency ls between ,fz and fs there is no tendency for the oscillator frequency to become stabilized, except at fz, which is higher than fr and separated therefrom by the desired difference frequency. It is control circuit. I5.

III on triode IIS. (that is, triode Hals permitted to amplify) and main discriminator It is allowed to assume its above explained command over the Auxiliary 'discriminator III therefore renders frequency control of oscillator II by main discriminator Id ineffective in the critical region where there is danger of the oscillator frequency becoming substantially stabilized at a frequency other than that which will produce the desired difference frequency. This is indicated as shaded in Figure Discriminator III, triode H3 and lead III thereby eectively valve the control action of discriminator It, specifically rendering it ineffective when the oscillator frequency is below fi.

' In order to prevent the undesired lock-in at fr discussed above, when scanning is not used, the

characteristic of discriminator III maybe modionly during the first considered condition of as- Y cending oscillator frequency from below f4 that corrective measures are required to insure reliable automatic frequency control at yall times.

According toriny invention, I render the output of discriminator It inoperable to prevent the increasing oscillator frequency from increasing above frequency f3. This is accomplished, in the preferred embodiment of the invention, by providing auxiliary discriminator I I I which is tuned to difference frequency Fa representing the difference frequency between oscillatorV frequencies frand f4, and may have the same characteristics as main discriminator I4. v

Whenever the frequency of oscillator II goes below f4 and the difference frequency is within the range of auxiliary discriminator III, the out put of auxiliary discriminator I I I is negative, and this causes grid I'I2 to become more negative. As

a result, the current through resistor I'I6 drops, i

the potential on leadml I'I becomes more negative and triode IIS, which is suitably biased, is rendered non-conductive. This insures that main discriminator Id has no control over the scanning action when the output of discriminator III is` negative, and lock-in when the oscillator frequency is below f4 is prevented.

When the frequency of oscillator II is above f4 and the difference frequency is within the range of auxiliary discriminator III the output of auxiliary discriminator III is positive. lDue to resistor III) thisv positive output has little or no effect in varying the output of triode II3 which remains substantially at the zero input condition. The same is` true when auxiliary discriminator III has no output. In both of these cases, no control effect is exerted by auxiliary discriminator ed'by suitable auxiliary circuit elements, toghave the form shown by the dash-dot line I32, which' produces zero output at the frequency fa. In this way, triode H3 blocks the tuning control between frequencies fa and fr also, so that no desired lockin at f1 can' occur. .l

Figure 21 illustrates a further embodiment of the invention similar to that of Figure `18 but wherein the output of the main discriminator is controlled by a suitable tuned circuit arrangement to prevent lock-in at an undesired frequency. The auxiliaryv discriminator III and control tube I I3 and its circuit are omitted inFig.

21, being replaced by the apparatus of Fig. 21 connected between leads 00 and II'l.

The output of intermediate frequency amplier I2 is connected by lead Iilll to a tuned circuit comprising coupled primary and secondary coils I33 and .|34 having parallel condensers |35 and I36 respectively. The secondary output passes through a suitable rectier I3`I and is connected by lead I38 to the grid of triode IIS. The remainder of the system of Fig. 21, although not shown in detail in Fig. 21, is the same as Figurel.

Secondary |34; I36 has such a resonant frequency that it delivers an appreciable output only when the frequency of oscillator II is below f4 and in the range indicated by shading in'Figure 20. Rectifier |31 passes only the negative part of the output of secondary i3d, I36. Thus, triode II9 being biased to be cut oi when grid IIB is made more negative, the tuned circuit valves the control action of vmain discriminator I4.

I have described above two frequency sensitive arrangements for blocking control action of main discriminator I4 for preventing undesired lock-in of the oscillator frequency. Any equivalent arrangement for accomplishing this purpose is within the scope of the invention. y

The auxiliary discriminator and tuned circuit controls above described and illustrated in Figures 18 and 21 may be equally well applied to any automatic' frequency control system, as for eXample where no scanning control circuit I5 is employed and the output of discriminatork I4 is directly connected to the frequency regulating means of oscillator II.

The invention illustrated in Figures 18-21 is of course not limited to the multivibrator type scanning control circuits earlier described in the application, but may comprise any suitable scan-` ning control. q

Either or both of the aboveV assumptions as to polarity of discriminator output or direction'of response of the controlled oscillator frequency to quency stabilization takes place with the oscillator frequency at f1. This is a manner of selecting a desired sequential order between the reference and oscillator frequencies.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that allmatter contained in the above description or shown in the accompanying drawings shall be-interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. Automatic frequency control apparatus for a variable frequency device comprising variably energizable tuning means for said device, means for positively cyclically varying energization of said tuning means for positively cyclically varying the operating frequency of said device over a predetermined band. means producing a signal corresponding in magnitude and in sense to variation of said operating frequency from a desired frequency condition,Y and means responsive to said signal for both controlling said cyclical variation and maintaining said operating frequency substantially at said desired frequency condition.

2. Automatic frequency control apparatus for a variable frequency device comprising reversible control means for cyclically varying the operating frequency of said device. means for producing a signal representing variation of said operating frequency from a desired frequency condition, said signal comprising a succession of electrical pulses, and means introducing said signal into said control means for modifying said cyclic operation, said pulses serving to cause reversal of said control means with changes in polarity of said signal.

3. Automatic frequency control apparatus for a high frequency device comprising means for cyclically varying the operating frequency of said device over a predetermined frequency band, means for producing a signal having polarity and magnitude respectively representing the sense and magnitude of variation of s aid operating frequency from a desired frequency condition, and means responsive to successive alternations of the polarity of said signal for both controlling said cyclical variation and maintaining said operating frequency substantially at said desired frequency condition.

4. The apparatus defined in claim 3, including means combining the outputs of said signal responsive -means and said frequency varying means for exercising said frequency control.

' 5. The automatic frequency control apparatus dened in claim 3, wherein said Signal is pulsed. 6. Automatic frequency control apparatus for substantially maintaining a'desired frequency difference between two high frequencysources comprising vacuum-tube oscillator means for cyclicall'y varying the operating frequency of one of said sources to repeatedly scan a predetermined frequency band, means producing a control signal representing departure from said desired frequency difference, and means responsive to change of polarity of said control signal for reversing said cyclical variation and for maintaining said desired frequency diiference.

7. Apparatus for automatically maintaining a high frequency device at or near a desired frequency comprising means for tuning said apparatus, multivibrator means connected for controlling said tuning means for cyclically varying the operating frequency of said device over a predetermined frequency range. and means responsive to the output frequency of said device for reversing the frequency controlaction of said multivibrator means to maintain said output frequency near or at said desired frequency.

8. Automatic frequency control apparatus for a high frequency device comprising means for tuning said device. scanning control means having a cyclically varying output, means for producing a control signal having variable magnitude and reversible polarity respectively representing deviation in magnitude and sense of the operating frequency of said device from a desired frequency, and means jointly responsive to said control signal and said cyclically varying output for maintaining said operating frequency substantially at said desired frequency.

9. In automatic frequency control apparatus for a velocity modulation device wherein an electron beam is projected through an ultra high frequency resonator ileld and returned therethrough by a reflector electrode. electro-mechanical tuning means for said resonator, electrical tuning means for said resonator embodying means for varying the potential of said rehector electrode, a source of electrical energy including a frequency discriminator and a multivibrator connected to produce a reversible cyclically varying output, and means connecting said cyclically varying output through long and short time constant circuits for energizing said electro-mechanical and electrical tuning means respectively.

10. An indicating system embodying a receiver for pulses of ultra high frequency energy, a local oscillator having an ultra high frequency output, means for mixing said receiver and local ,oscillator outputs for obtaining a desired difference frequency, and means for automatically maintaining said difference frequency at or near a desired value comprising means including a multivibrator for cyclically varyin'g the operating frequency of said local oscillator over a predetermined frequency band, and means responsive to variation of said difference frequency through said desired frequency for reversing said multivibrator. A

l1. Automatic frequency control apparatus for maintaining a substantially xed frequency difference between two high frequency sources comprising. discriminator means responsive to said frequency difference and providing an output proportional to and sensed in accordance with deviation of said frequency difference from said fixed value, scanning means having a cyclically varying output, and tuning means for one of said sources jointly responsive to said outputs.

12. Automatic frequency control apparatus for a variable frequency device comprising reversible control means for cyclically varying the operating frequency of said device, means for 'producing a signal having a sense representing the direction of variation of said operating frequency from a desired frequency condition, and means introducing said signal into said control means for modifying said cyclical operation, said signal serving to cause reversal of said control means with changes in polarity of said signal.

13. Automatic frequency control apparatus comprising a source of continuous electromag- 2l netic wave energy, heterodyning means connected to said source for producing -a difference frequency to be controlled, means for pulsing said difference frequency, and means including a frequency discriminator and a multivibrator connected to produce a reversible-polarity variablemagnitude control signal in accordance with deviations of said difference frequency from a desired.` frequencyf'value and for controlling the output frequency of said heterodyning means whereby said difference frequency is transformed to and` maintained substantially at said Y.

desired frequency value.

14. Automatic frequency control apparatus for a frequency-variable device comprising frequency regulation means, scanning means having a variable output voltage oscillating at a prev t the polarity of said control signal for substantially decreasing theperiod of said variable output voltage and of said cyclic frequency variation of said device.

15. Automatic frequency control apparatus for maintaining a desired frequency difference between two sources of high frequency electromagnetic energy comprising means for cyclically varying the operating frequency of one of said sources to repeatedly scan a predetermined frequency band, means producing a control signal representing departure from said desired frequency difference, and means responsive to change of polarity of said control signal for reversing said cyclical variation and for substantially maintaining said desired frequency difference.

16. Apparatus for automatically maintaining a high frequency device substantially at a desired frequency comprising means for tuning said apparatus, means connected for controlling said tuning means for cyclically varying the operating frequencyof said device over a predetermined frequency range, and means responsive to the output frequency of said device for reversing the frequency control action of said control means to maintain said output frequency substantially at said desired frequency.

17. High frequency apparatus comprising a receiver for e high frequency electromagnetic wave energy, a local oscillator, means for vmixing 22- the outputs oi said receiver and said local oscillator for obtaining a desired difference frequency, means for automatically maintaining said dierence frequency substantially at a desired value comprising a multivibrator for cyclically Y `varying the operating frequency of said local os,-

cillator and means responsive to variation of said difference frequency through said desired-fre quency, forreversingthe polarity ofsaid multivibrator.

18. Automatic frequency' control apparatus comprising a high frequencydevice whose frequency is to be controlled, means for deriving a signal corresponding to deviation of .the frequency of said device from a desired value, and means for controlling the frequency of said device, said last-named means comprising an oscillator Y adapted to produce an output of a given frequency, an integrating circuit for integrating said output, said circuit having a time constant at least of the order of magnitude of the period of said oscillator output, means coupling said circuit to said device to control the frequency thereof by said integrating output, whereby said control frequency is cyclically varied, and means responsive to .said signal for materially increasing said oscillator frequency whereby said integrated output is maintained substantially constant to keep said controlledv frequency substantially at said desired frequency.

EDWARD L. GINZTON.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,847,160 /Ailel Mar. 1, 1932 l1,934,400 .Bollman Nov. 7, 1933 2,012,603 Fuchs, Aug. 27, 1935 2,098,331 VBowman Nov. 9, 1937 2,162,335 Jacob June 13, 1939 2,173,902 Gerth Sept. 26, 1939 2,203,750 Sherman June 11, 1940 2,242,249 lVIay 20, 1941 2,242,275 Varian May 20, 1941 2,245,627 Varian June 17, 1941 2,254,601 Felch Sept. 2, 1941 2,262,147 owsley Nov. 11, 1941 2,284,266 Bellescize May 26J 1942 A2,287,925 White June 30,1942

2,294,942 Varian sept. a, 1942 I 2,311,658 Hansen' Feb. 23,1943 2,326,737 Andrews Aug. 17, 1943 2,404,568 i Dow July 23,1946

numbered patent requiring' correction as follows:

.Cotglcate of Gorrection i Patent No. 2,434,294.' y January 13, 194s.

EDWRD L. GINzToN Itis herebyr certified tht error appears in the Ciniinted pcic5a5tiofn ofthe ahove o umn ine or scanmng read spacing; and that the said'Letters Patent should be read with this correction therein that the same may conform lto the record of. the case in the Patent Oce.

" Signed and sealed this 1th day of Maty, A. 1948;

THOMAS F. 'MimPHm Assistant onmasoner of Patents. 

